Thank you very much Soilworker- that was very helpful, actually.
It has helped to clarify things somewhat; I
did understand (sort of!) the gamut issues with CMYK reproduction, it just does my head in that the various bits of software have such wildly varying ideas on how to match each colour. I just want to be able to provide the information for my client properly without them finding down the line that I've done it 'wrong'.
*sigh*
Thanks again.